A local loop is the portion of a telecommunications system which connects a local exchange to the subscribers. Conventionally this is done by means of wires or optical fibres, but a radio link is another possibility. If subscriber stations are mobile, a radio link has to be used, but there are advantages even for static subscribers. For example, lines can be installed, changed and maintained without having to dig up pavements or erect and climb telegraph poles. A radio local loop involves a base station which transmits and receives radio signals at a location which is determined by signal strengths and reception conditions and is not necessarily near to the local telephone exchange, and subscriber units located at subscriber premises, each of which includes an antenna whose location is determined by considerations of signal reception and transmission and a connection to the subscriber's equipment (telephone, fax, computer etc.). Subscribers attempting access to the network will usually have to prove their identity via some means (e.g. electronic serial number, subscriber PIN, smart card key inserted into subscriber unit etc). However, these mechansims can become known to third parties who could then "impersonate" a valid subscriber to gain access via the air interface using representative (or stolen) equipment.